Process control systems are used to control industrial processes such as chemical, oil and gas, paper and pulp and electric power generation processes.
These systems are typically computer systems and may comprise one or more process control devices, for instance in the form of one or more servers, communicating with a number of field devices. In such control systems the field devices measure properties of the process being controlled, supplies the measurements to the process control device, while the process control device supplies the field devices with control commands for controlling the process. The field devices may in such a system generate a lot of data which is supplied to the process control device.
It has recently been of interest to provide such field devices as nodes in industrial wireless communication networks. There exist a number of industrial wireless communication standards that are possible to use for industrial control purposes.
Examples of such standards are ZigBee, WirelessHart, ISA100, WIA-PA and Bluetooth. There also exist some WLAN based sensor networks.
The field devices then form nodes in an industrial wireless network and communicate with a gateway that is connected to a control device, such as a server or a special purpose controller.
However, if there are a lot of such nodes, there may be a problem that the wireless network is filled with data, which slows down the transmission from the nodes and thus slows down the control.
Furthermore, the process control system may be placed in a harsh environment, which makes transmission in the wireless network problematic.
The most common solution in available industrial wireless sensor networks to provide reliable communication is to use mesh networking, i.e., each wireless communication device is connected to two or more other devices in order to somewhat guarantee reliable communication. One way of improving the reliability in a harsh environment is to use error correction codes such as automatic repeat request (ARQ) and having alternative links for each node. To avoid having packets wandering around in the network forever, a hop counter or a time-to-live (TTL) field may be used. To achieve reliable communication the hop-counter or TTL may be configured in the range of more than 10 hops and with a TTL of several minutes to allow packets to traverse alternative paths with several retransmissions on each link. This value is however typically common for the whole wireless network, which may be problematic if the wireless network is also used for another functionality than control that has another delay requirement, where one such other functionality is safety or protection.
It is thus sometimes of interest to simultaneously use a wireless network for different types of functions having different delay requirements, such as real-time traffic and best effort traffic. These differing delay requirements cannot both be met at the same time using the TTL principle.
Another way of improving the reliability in a harsh environment is to use forward error correction (FEC). In available industrial wireless sensor networks ARQ is employed while FEC has not been implemented since that is typically implemented on the physical layer and it has previously been discussed that it consumes a lot of power for encoding and decoding.
Various other schemes have been proposed in relation to the limiting of data in industrial wireless systems, where one scheme is described in US 2012/0063339. Here packets are discarded based on the determination of an expected delay through a sensor network and a pre-determined allowable value.
However, there is still room for improvement in the field.